Hyunah Kwon

Three-Dimensional Metal-Oxide Nanohelix Arrays Fabricated by Oblique Angle Deposition: Fabrication, Properties, and Applications.

Three-dimensional (3D) nanostructured thin films have attracted great attention due to their novel physical, optical, and chemical properties, providing tremendous possibilities for future multifunctional systems and for exploring new physical phenomena. Among various techniques to fabricate 3D nanostructures, oblique angle deposition (OAD) is a very promising method for producing arrays of a variety of 3D nanostructures with excellent controllability, reproducibility, low cost, and compatibility with modern micro-electronic processes. This article presents a comprehensive overview of the principle of OAD, and unique structural and optical properties of OAD-fabricated thin films including excellent crystallinity, accurate tunability of refractive indices, and strong light scattering effect which can be utilized to remarkably enhance performances of various systems such as antireflection coatings, optical filters, photoelectrodes for solar-energy-harvesting cells, and sensing layers for various sensors.

A study of the role of various reactions on the density distribution of hydrogen, silylene, and silyl in SiH4/H2 plasma discharges

Fluid model has been used to study the effect of pressure on the distribution of hydrogen (H), silylene (SiH2), and silyl (SiH3) in hydrogen silane plasma discharges used for deposition of hydrogenated microcrystalline silicon (μc-Si:H) or hydrogenated amorphous silicon (a-Si:H) thin films for solar cells. Time averaged reaction rates have been calculated to study the influence of various reactions on the density distributions of hydrogen, silylene, and silyl. Change in the distributions of hydrogen and silylene from bell shaped distribution at low pressure (1 Torr) to double humped distribution at high pressure (5 Torr) is explained with the help of time averaged reaction rates. Important reactions have been identified that contribute to the production and consumption of hydrogen (H), silylene (SiH2), and silyl (SiH3). The hydrogen consumption reactions SiH4 + H → SiH3 + H2 and SiH3 + H → SiH2 + H2 are found to play a central role in deciding the distribution of hydrogen. On the other hand, silylene cons...

Wavelength-dependent visible light response in vertically aligned nanohelical TiO2-based Schottky diodes

Enhancements in photocatalytic performance under visible light have been reported by noble metal functionalization on nanostructured TiO2; however, the non-uniform and discrete distribution of metal nanoparticles on the TiO2 surface makes it difficult to directly clarify the optical and electrical mechanisms. Here, we investigate the light absorption and the charge separation at the metal/TiO2 Schottky junctions by using a unique device architecture with an array of TiO2 nanohelixes (NHs) forming Schottky junctions both with Au-top and Pt-bottom electrodes. Wavelength-dependent photocurrent measurements through the Pt/TiO2 NHs/Au structures revealed that the origin of the visible light absorption and the separation of photogenerated carriers is the internal photoemission at the metal/nanostructured TiO2 Schottky junctions. In addition, a huge persistent photoconductivity was observed by the time-dependent photocurrent measurement, implying a long lifetime of the photogenerated carriers before recombination. We believe that the results help one to understand the role of metal functionalization on TiO2 and hence to enhance the photocatalytic efficiency by utilizing appropriately designed Schottky junctions.Enhancements in photocatalytic performance under visible light have been reported by noble metal functionalization on nanostructured TiO2; however, the non-uniform and discrete distribution of metal nanoparticles on the TiO2 surface makes it difficult to directly clarify the optical and electrical mechanisms. Here, we investigate the light absorption and the charge separation at the metal/TiO2 Schottky junctions by using a unique device architecture with an array of TiO2 nanohelixes (NHs) forming Schottky junctions both with Au-top and Pt-bottom electrodes. Wavelength-dependent photocurrent measurements through the Pt/TiO2 NHs/Au structures revealed that the origin of the visible light absorption and the separation of photogenerated carriers is the internal photoemission at the metal/nanostructured TiO2 Schottky junctions. In addition, a huge persistent photoconductivity was observed by the time-dependent photocurrent measurement, implying a long lifetime of the photogenerated carriers before recombinatio...

P2.7.9 TiO2 Nanohelix Gas Sensors with Enhanced Performance and Potential Application as a Building Block of Electronic Noses

We present TiO2 nanohelix gas sensors with top-and-bottom electrodes configuration fabricated by oblique angle deposition (OAD) that is very simple, easy, and compatible with conventional microfabrication processes. The combination of large surface-to-volume ratio, extremely small size (~ 22 nm) with near single crystallinity of TiO2 nanohelices, together with the top-and-bottom electrodes configuration results in a huge enhancement in gas sensing performances including ~10 times higher sensitivity, ~5 times lower detection limit, and much faster response time than the conventional thin film device. By employing this simple, cheap, and easy method, e-nose, consisting of each sensor devices with various nanostructures and metal-oxides, can be realized, having the large freedom of tuning the sensitivity of each device.